Mo Cain on mon 24 mar 97
It is not my intent to start an ongoing debate but does anyone have an
easily understood rationale for sometimes including B2O3 in the RO
column for the Seger Unity analysis and sometimes including it in the
RO2O3 column? It seems to me that for any given batch recipe analysed
either way (B2O3 a flux and B2O3 not a flux) it would lead to different
theoretical formulae and presumably different limits for each ingredient
yet the actual glaze is the same in either case. Is this a big deal or
am I barking up the wrong tree? I am relatively new at this analysis
lark and I am somewhat bemused by the fact that batch recipes are
carefully weighed out in grams to two decimal places and the unity
formula calculated often to three decimal places and yet to get the
glaze to work I sometimes have to add a little bit of this or subtract a
little bit of that and generally fiddle about and usually ending up with
something entirely different. From my discrete lurking position I
recognise that there are some mighty sharp folks out there in Clayland
so I anticipate some further insight into what for me is a mystery.
ta very much... mo the brit in Atlanta where spring has really sprung.
M Richens on thu 27 mar 97
In article <33356DC9.3A2F@bellsouth.net>, Mo Cain
writes
>----------------------------Original message----------------------------
>It is not my intent to start an ongoing debate but does anyone have an
>easily understood rationale for sometimes including B2O3 in the RO
>column for the Seger Unity analysis and sometimes including it in the
>RO2O3 column? It seems to me that for any given batch recipe analysed
>either way (B2O3 a flux and B2O3 not a flux) it would lead to different
>theoretical formulae and presumably different limits for each ingredient
>yet the actual glaze is the same in either case. Is this a big deal or
>am I barking up the wrong tree? I am relatively new at this analysis
>lark and I am somewhat bemused by the fact that batch recipes are
>carefully weighed out in grams to two decimal places and the unity
>formula calculated often to three decimal places and yet to get the
>glaze to work I sometimes have to add a little bit of this or subtract a
>little bit of that and generally fiddle about and usually ending up with
>something entirely different. From my discrete lurking position I
>recognise that there are some mighty sharp folks out there in Clayland
>so I anticipate some further insight into what for me is a mystery.
>
>ta very much... mo the brit in Atlanta where spring has really sprung.
Hi Mo,
I'm guessing a touch here. The unity formula can be seen as
Alkali Amphoteric Acid
X2O/XO X2O3 XO2
or
Fluxes Modifiers Glass Formers
This latter as used in the excellent book on 'Ceramics Glaze Technology'
by J R Taylor ad A C Bull.
In the latter case B2O3 would go with the glass formers as it is second
to silica in that respect. It is also a great flux and has the advantage
of having a very low expansion coefficient. This means that if you want
to pull down the firing temperature but keep the glass formers up then
substitute B2O3 for SiO2. This makes much more sense to me than
including it with Al2O3 as this disguises its true nature.
Then, however, you need to think about TiO2 as it is a network modifier
and not a glass former. (things can get complex as combinations of TiO2
and Al2O3 can contribute to the glass structure alongside of Silica..)
Vague or What? :-}.
I would plump for putting it with the Silica but what do I know. I
designed fully fritted enamels and we didn't use unity formulae.... just
oxide and mineral.
Max
--
Max Richens max@richens.demon.co.uk +44 (0) 1925756241
Enamel Consultant - Ceramist - Analyst programmer
Software for Batch Formulation and Millroom control.
Cheshire Hash House Harriers.
Gavin Stairs on thu 27 mar 97
Mo Cain wrote...
>>It is not my intent to start an ongoing debate but does anyone have an
>>easily understood rationale for sometimes including B2O3 in the RO
>>column for the Seger Unity analysis and sometimes including it in the
>>RO2O3 column? ...
Max Richens wrote:
....
>In the latter case B2O3 would go with the glass formers as it is second
>to silica in that respect. It is also a great flux and has the advantage
>of having a very low expansion coefficient. This means that if you want
>to pull down the firing temperature but keep the glass formers up then
>substitute B2O3 for SiO2. This makes much more sense to me than
>including it with Al2O3 as this disguises its true nature.
>Then, however, you need to think about TiO2 as it is a network modifier
>and not a glass former. (things can get complex as combinations of TiO2
>and Al2O3 can contribute to the glass structure alongside of Silica..)
>Vague or What? :-}.
....
Vague or what indeed. I was sitting down with Ron Roy a little while ago to
try to understand B2O3. If you could look at a few single axis phase
diagrams you woulld be able to follow this better, but... It seems that
B2O3 does not form a crystalline phase, even as a pure material. It is
glass from the get go. So on that basis, it is clearly a glass former.
Should be with SiO2. But...
It doesn't form a true eutectic with silica (alumina?): the minimum melting
point is at pure B2O3. What the... Well, I guess that means it is really a
flux. File it under XO. But...
It is homologous with Al2O3. File it under X2O3. Hmm...
I think the lesson is that the three categories, while undeniably useful,
are not rigorous. Can you say heuristic? It means I do it because it works
most of the time. The few exceptions I mumble over, and sweep under the
rug. Any rug that works (ie., XO2 or XO: probably not X2O3, paradoxically).
Don't fret; frit. Ok, I'm sorry about that one.
Gavin
M Richens on sat 29 mar 97
>
>It doesn't form a true eutectic with silica (alumina?): the minimum melting
>point is at pure B2O3. What the... Well, I guess that means it is really a
>flux. File it under XO. But...
Doesn't this just mean it makes a full range solid solution (cf Alcohol
and Water) or am I simplistic or misremembering books?
>
>It is homologous with Al2O3. File it under X2O3. Hmm...
>
>I think the lesson is that the three categories, while undeniably useful,
>are not rigorous. Can you say heuristic? It means I do it because it works
>most of the time. The few exceptions I mumble over, and sweep under the
>rug. Any rug that works (ie., XO2 or XO: probably not X2O3, paradoxically).
>Don't fret; frit. Ok, I'm sorry about that one.
>
Ouch!!
>Gavin
Thanks for the arguements. Much better than I could have put it. but
doesn't it make glasses fascinating.
Its intriguing the way having the same Oxide formula but changing the
mineral makeup can alter the properties of the final glass (for instance
to change the rate of leaching). With enamels if you put the F2 in as
cryolite instead of fluorspar while keeping everything else the same
then the final frit was much less acid resistant to citric as well as
Sulphuric possibly the contribution of the Al as feldspar gave better
structure. I wish I had had better equipment and less commercial
pressures so that I could have investigated further at the time.
Cheers
Max
--
Max Richens max@richens.demon.co.uk
+44 (0) 1925756241
Enamel Consultant - Ceramist - Analyst programmer
Software for Batch Formulation and Millroom control.
Gavin Stairs on sun 30 mar 97
At 08:57 AM 29/03/97 EST, Max Richens wrote:
....
>>It doesn't form a true eutectic with silica (alumina?): the minimum melting
>>point is at pure B2O3. What the... Well, I guess that means it is really a
>>flux. File it under XO. But...
>Doesn't this just mean it makes a full range solid solution (cf Alcohol
>and Water) or am I simplistic or misremembering books?
....
Yes. It is really the low temperature glassy state at 100% B2O3 that is
most peculiar. My argument here is very loose, just to point vaguely at the
strong flux column. Chemistry is a matter of competing minima, and most of
what we think we know in the way of easy rules seems to break down in the
crunch.
....
>Its intriguing the way having the same Oxide formula but changing the
>mineral makeup can alter the properties of the final glass (for instance
>to change the rate of leaching). ...
Yes indeed. A particle a micron in size is still many thousands or millions
of molecules. If, in the midst of the glass, a particle many times less
than a micron still survives, then the system is still susceptible to attack
on the properties of the original material. Or perhaps worse, at the
stressed region at the particle boundary. Even if the material is wholly
glassy, it is still necessary to transport material the few microns from one
place to another in the melt, in order to achieve a uniform melt at the
atomic/ionic/molecular level. A practical glaze or enamel may not be
perfectly homogeneous, in fact probably isn't. And in the case of a
material with a volatile radical attached, it is necessary to transport both
the oxidizer and the volatile radical, not only from/to the air boundary,
but across it. All of these diffusion reactions take time, high
temperatures, and the right concentrations of reagents to go to completion.
There is ample evidence that these conditions are not met, in general, and
that practical glazes are not the simple, homogeneous, glassy melts that we
naively expect. At the submicroscopic level (under 0.5 microns or so) there
may be lots of variation, even in an apparently smooth and homogeneous glaze
coat. Of course, if you begin with a visibly variegated glaze, you know
from the start that it is not uniform.
All this, and we haven't yet begun to cool!
With today's sophisticated tools, like scanning electron microscopes,
scanning force microscopes, etc., it should be possible to investigate these
matters at the molecular scale. Unfortunately, esthetic concerns do not
rank high on the priorities of materials research departments at
universities and materials firms, where such equipment may be available. I
am beginning to think that we need an institute of science and technology
for esthetic ceramics, in order to win access to such facilities. Perhaps
to promote visiting appointments to particular labs, or even to acquire and
operate such equipment at some far thinking university, or to raise funds
for independent studies.
Gavin
Gavin Stairs Hi Dannon
http://isis.physics.utoronto.ca/
| |
|
